1. Field of the Invention
The present invention relates to a window glass antenna device which has, as a reception antenna or part of a reception antenna, defrosting heater wires disposed on a window glass panel of an automobile.
2. Description of the Prior Art
Use of defrosting heater wires on a window glass panel of an automobile as a reception antenna or part of a reception antenna requires that the heater wires be of high impedance with respect to a heater power supply or an automobile body as ground. The heater wires are therefore supplied with a heating current through a choke coil.
The choke coil tends to be large in size because the heating current is of a relatively large magnitude ranging from several amperes to several tens of amperes. In view of such a problem, it has been customary to wind choke coil windings, to be connected respectively to positive and negative terminals, on one core by way of bifilar winding, thus preventing the core from being magnetically saturated by a heating direct current, keeping the heater wires at high impedance, and reducing the size of the choke coil. However, the efforts to reduce the size of the choke coil through bifilar winding are subject to limitations because the diameter of windings cannot be reduced.
FIGS. 7(a), 7(b), and 8 of the accompanying drawings show conventional window glass antenna devices.
FIG. 7(a) illustrates a known single-sided feeding structure for feeding heater wires, and FIG. 7(b) illustrates a known double-sided feeding structure for feeding heater wires.
The window glass antenna device with the single-sided feeding structure shown in FIG. 7(a) has a plurality of heater wires HW disposed horizontally across a window glass panel 65 and divided into upper and lower groups. Bus bars RB1, RB2 serving as feeder terminals for supplying a heating current to the heater wires HW are positioned on the right-hand side, for example, of the window glass panel 65, and a returning bus bar BB for returning the heating current is positioned on the left-hand side of the window glass panel 65.
Since the bus bars RB1, RB2 are located on one side, i.e., the right-hand side, of the window glass panel 65, connector wires 67, 68 interconnecting the bus bars RB1, RB2 to a choke coil CH may be relatively short. However, the resistance per unit length of each of the heater wires HW has to be low in order for the heater wires HW to be supplied with a predetermined heating current under an automobile battery voltage of 12 volts, for example, because each of the heater wires HW is relatively long.
As shown in FIG. 9 of the accompanying drawings, the heater wires HW are manufactured by printing a silver paste 65a to a certain thickness on the window glass panel 65 as a base, drying and baking the silver paste 65a, plating a copper layer 65a on the silver paste 65a, and then plating a chromium layer 65c on the copper layer 65a to increase the mechanical strength of the heater on the window glass panel 65. This manufacturing process is relatively complex and entails an increase in the cost of the antenna device. The plating steps require a large investment to be made in building an installation for processing waste solutions.
The window glass antenna device with the double-sided feeding structure shown in FIG. 7(b) has a plurality of heater wires HW disposed horizontally across a window glass panel 75 and a pair of bus bars LB, RB disposed one on each side of the heater wires HW. The bus bars LB, RB are connected through respective connector wires 77, 78 to a choke coil CH that is positioned near the bus bar RB, for example. The choke CH is connected to a battery BAT as a heating power supply and a capacitor C for removing noise from a heating current supplied from the battery BAT. Since the choke CH is positioned near the bus bar RB, the connector wire 77 is shorter and the connector wire 78 is longer. With the connector wire 78 being longer, the coupling capacitance between the connector wire 78 and an automobile body such as a metallic window frame as ground is increased, resulting in an impedance reduction. Since the connector wires 77, 78 connected to the respective bus bars LB, RB are of different lengths, their impedances with respect to the automobile body are unbalanced. The unbalanced impedances are responsible for a reduction in the reception sensitivity of the antenna and a change in the directivity of the antenna. Inasmuch as the heater wires HW are often used as an antenna for receiving AM broadcasts, any change in the position of the connector wire 78 is undesirable as it would cause the reception sensitivity to vary.
The window glass antenna device shown in FIG. 8 corresponds to one of the typical embodiments disclosed in Japanese laid-open utility model publication No. 3-117918.
According to the disclosed window glass antennas, at least one of a pair of bus bars on both sides of a plurality of heater wires extends substantially horizontally toward the other bus bar such that signal pickup terminals of these bus bars are positioned closely to each other.
As shown in FIG. 8, the window glass antenna comprises a plurality of heater wires HW on a window glass panel 85 and two bus bars LB, RB disposed one on each side of the heater wires HW. Bus bar extensions LBE, RBE extend horizontally from the respective lower ends of the bus bars LB, RB toward the center of the window glass panel 85, and have respective distal ends P1, P2 connected to a choke coil CH. Since the ends P1, P2 or signal pickup terminals of the bus bars LB, RB are positioned clearly to each other, they can easily be connected to the choke coil CH.
If the bus bar extensions LBE, RBE are formed of electrically conductive frit without plating, then since the resistance of the bus bar extensions LBE, RBE cannot be smaller than 1 ohm per length of 10 mm and width of 1 mm, these bus bar extensions LBE, RBE develop an unwanted voltage drop and are heated when the heater wires HW are energized. Conversely, if the bus bar extensions LBE, RBE are of the structure shown in FIG. 9, then the antenna device on the window glass panel 85 is complex in structure and expensive to manufacture.
FIG. 10 of the accompanying drawings illustrates another conventional window glass antenna device.
As shown in FIG. 10, the window glass antenna device comprises a first heater 92 disposed on a window glass panel 95 and having bus bars LB1, RB1 on respective ends thereof, a second heater 93 disposed on the window glass panel 95 and having bus bars LB2, RB2 on respective ends thereof, and an antenna 94 disposed on the window glass panel 95. The first heater 92, which is composed of heater wires HW, is connected to a positive terminal of a heating power supply 98 through a first winding AL1 of a choke coil 96 and also to an automobile body as ground through a first winding BL1 of a choke coil 97. The second heater 93, which is composed of heater wires HW and whose resistance is substantially the same as the resistance of the first heater 92, is connected to the positive terminal of the heating power supply 98 through a second winding BL2 of the choke coil 97 and also to the automobile body through a second winding AL2 of the choke coil 96. The window glass antenna device is arranged such that currents through the respective first and second heaters 92, 93 cancel out magnetic fluxes generated in the cores of the choke coils 96, 97.
The numbers, lengths, and thicknesses of the heater wires HW are designed such that the resistances of the first and second heaters 92, 93 are equal to each other. However, in the mass production of the window glass antenna devices, it is difficult to equalize the resistances of the first and second heaters 92, 93 exactly with each other, and actually the resistances of the first and second heaters 92, 93 differ from each other. Consequently, currents flowing through the first and second heaters 92, 93 also differ from each other, and hence the magnetic fluxes in the cores of the choke coils 96, 97 are not canceled out due to the different currents flowing through the first and second windings of each of the choke coils 96, 97. As a result, the choke coils 96, 97 have poor inductance characteristics. FIG. 11 of the accompanying drawings shows a characteristic curve representing the relationship between the inductance of each choke coil and the difference between the currents flowing through the choke coil. It can be seen from FIG. 11 that the inductance (.mu.H) of the choke coil greatly decreases even with a small current difference (A). The reduction in the choke inductance brings about poor antenna characteristics.